This invention relates to low capacitance field emission triodes and to a method for fabricating such triodes.
Field emission devices (FEDs) have been known for many years. However, FEDs typically use a structure Commonly known as a Spindt tip for the emitter. However, Spindt tips require a very small feature size in triodes to provide the desired emission and control of the emitted electrons. This very small site is very difficult to achieve and can result in serious damage to the tip during high current emission or if it is not uniformly constructed.
Also, the gate in prior field emission triodes is generally fabricated by surrounding the Spindt tip with a dielectric material and then depositing a metal layer on the dielectric material. An opening is etched through the metal layer to define an edge of the metal layer equa-instant from the extreme tip of the Spindt tip. Some of The dielectric is then removed to form a well surrounding the Spindt tip. The centering of the edge around the extreme tip and the distance of the edge from the extreme tip is critical to the operation and draws electrons from the Spindt tip toward an anode disposed remotely beyond the metal layer. In addition to the problem of centering the edge around the tip at the correct distance, one problem that occurs from this arrangement is that the electrons have a tendency to disperse rather widely and must be focused in some fashion into the desired spot size at the anode. Another problem that occurs in Spindt tips is the need to provide a ballast resistor with each tip. Ballast resistors are relatively difficult to incorporate into an array of Spindt tips and require additional process steps.
In this prior art structure, the metal layer and the dielectric layer combine to form a relatively high capacitance. Further, the gate to emitter separation is very close, which results in additional capacitance. These capacitances combine in an array of Spindt tips to produce an overall capacitance for the device which becomes a serious problem when using high speed or high frequency signals in the displays.
Accordingly, it is highly desirable to provide a device which overcomes these problems and a method of fabricating the device.